Conventional interferometer spectrometers are based on the idea of wavefront separation into two beams and introduction of an optical path difference (hereinafter "OPD") between such beams. The change in OPD causes modulation of light intensity due to interference between the two beams. Each optical wavelength present in the input light generates its own modulation frequency. Thus, the spectral content of the input light can be decoded by using Fourier transform (hereinafter "FT").
In conventional polarization interferometer spectrometers, an OPD is introduced between two rays with orthogonal polarization directions inside the double-refractive crystal. The waves corresponding to ordinary and extraordinary polarizations separate upon incidence on the crystal and travel with different velocities. After passing through the crystal, the rays exhibit a phase delay between them, which is proportional to crystal thickness. The two rays then interfere with each other after passing through a polarizing analyzer. The resulting intensity variations, which bear the signature of presented spectral components, are transformed or converted into an electrical signal by a photodetector. The electrical signal is thereby recorded for analysis.
To restore spectral components of the input light by using FT, the signal should be sampled at least twice of the frequency band of the interference signal. From the theory of FT, it follows that in order to resolve the optical wavelength .lambda. to the accuracy .delta..lambda., the total accumulated OPD .DELTA. should satisfy the condition .delta..lambda.=.lambda..sup.2 /.DELTA.. For example, if required resolution .delta..lambda.=0.5 nm at .lambda.=500 nm, then A =0.5 mm.
There are various known methods of generating OPD by means of double-refractive or birefringent crystals. One example is a Soleil compensator. The Soleil compensator comprises two complementing optical wedges and a plane-parallel plate, made of a double-refractive material, positioned at the normal incident angle. The optical axes of both wedges and the plate lay in the surface plane and are perpendicular to each other. When one of the wedges is sliding along the dividing diagonal face across the beam, an OPD is introduced between rays with orthogonal polarization directions. Examples of polarizing interferometer spectrometers utilizing this principal are disclosed in U.S. Pat. No. 3,849,001 issued to Inoue et al. and U.S. Pat. No. 5,157,458 issued to Wagner et al.
These polarization interferometer spectrometers have significant advantages in comparison to the ones based on a double-beam type interferometer such as a Michelson or Mach-Zehnder interferometer. One advantage comes from the fact that partial beams with orthogonal polarization directions share a common optical path. As a result imperfections of optical materials and surfaces as well as mechanical vibrations have significantly less influence on amplitude of the interference signal. This allows extending the useful range of polarization interferometer spectrometers into visible and near infrared wavelength ranges. However, these polarization interferometer spectrometers have their own technical limitations in practical applications due to their requirement that the traverse movement of the optical wedge must be highly stable and linear.
U.S. Pat. No. 5,781,293 issued to Padgett et al. (hereinafter "Padgett") describes an invention that overcomes the limitations associated with the above described polarization interferometer spectrometers. Padgett reveals a FT spectrometer that employs no moving parts. Padgett teaches that a double-wedge element made of a birefringent material such as a Wollaston prism may be used to introduce a variable OPD between rays with orthogonal polarization directions. The OPD is distributed linearly in a plane across an optical beam. An array of photosensitive elements placed across the beam captures the resulting spatial interference pattern, which uniquely corresponds to the spectral content of the input light. The pattern is analyzed by means of Fast Fourier transform (FFT). Again, the presence of the common optical path in this design significantly reduces effect of surface imperfections and mechanical vibrations and allows measurements to be taken in the ultra-violet, visible and near infrared regions. The absence of moving parts simplifies the design and reduces spectrum acquisition time to millisecond range.
However, the Padgett spectrometer also has technical limitations. For example, the dynamic range of an array of photosensitive elements is typically lower then that of a single photodetector, due to light scattering and current bleeding from the neighbor pixels. The number of elements in the array and spacing between them also put limit on the spectral resolution. Commercially available spectrometers of this type are known to have spectral resolution in the range of 100 cm.sup.-1. That resolution may be insufficient for the majority of analytical and research applications.
Other conventional polarizing interferometers, primarily used for biological microscopy applications, utilize a slab of double-refractive material with its optical axes perpendicular to faces of the slab. Phase differences between ordinary and extraordinary rays are introduced by tilting the slab, and the OPD value is calculated from the tilting angle.
A common problem associated with polarizing interferometer spectrometers is the dependence of a material birefringence on wavelength of the input light. As a result, the OPD acquired for reference light would be different for some other spectral line. Because of that fact, the FFT algorithm is no longer applicable directly, and more general and slow digital FT algorithm is required for a high-resolution spectrum acquisition.
Another common limiting factor for the described interferometer spectrometers is a reduced throughput when such interferometer spectrometers are used in connection with a coupled optical fiber for delivering input light from a source. The cause of reduced light throughput is the dependence of refraction index from the angle of propagation in a double-refractive material. To maintain high fringes visibility, the phase variation .delta..DELTA. over entire cone of collinated light should not exceed .pi./4. For example, if the shortest wave in the spectrum .lambda.=200 nm, then .delta..DELTA..ltoreq.50 nm. As a result, to maintain synchronous phase variations across the beam, the light should be confined within small collimating angle. The product of the beam cross-section over acceptable light collimating angle defines the optical throughput or etendue of the system and is a limiting factor in the spectrometer throughput.
In summary, the known conventional designs of polarizing interferometer spectrometers with double-refractive elements, suffer from one or several of the following drawbacks: [1] low spectral resolution, [2] low dynamic range, [3] reduced optical throughput and [4] requirement for high stability of linear traverse stage.
Therefore, it is a general object of the present invention to provide an instrument and a method for determining the spectral content of the input light that address those above disadvantages.
It is another object of the present invention to provide an instrument that is portable or fits in a small size package.
It is another object of the present invention to provide an instrument that is simple and inexpensive to manufacture and thus is cost-effective.